The first vibrational spectroscopic analysis of a novel complex between a trans-tetraazacyclooctadeca-1,10-diene and nickel(II) perchlorate is reported. The molecular structure of Ni(trans[18]diene N 4 )(ClO 4 ) 2 .2H 2 O is centrosymmetric and evidence is found in the vibrational spectra for a delocalization of the two (C N) bonds through the planar NiN 4 ring system; this is a different structural situation to the NiN 4 system in nickel porphins where the four NiN bonds are equivalent. Comparison is made with the vibrational spectra of analogous porphin-metal complexes.